The present invention relates to the field of assembling an aircraft's primary structures. More specifically, the invention relates to a method of assembling a floor structure in a section of aircraft fuselage whose cockpit has been constructed previously.
An aircraft is made from an assembly of elements that play a role in forming the primary structure.
A fuselage is a special case of a primary structure comprising a body structure 2 and a floor 4, and is produced, for example, by assembling two or more sections. Generally, for body structure 2, each section is constructed from several stiffened panels assembled together to form a layer. Said panels comprise a skin 21 in the form of a metal or composite sheet formed according to the required profile, longitudinal stiffeners, known as stringers 25, fixed to the skin 21 by riveting, welding, bonding or co-firing depending on the nature of the materials present, and transverse stiffeners, known as frames 23, connected to the skin 21 and stringers 25 by riveting, welding, bonding or co-firing. The frames 23 are positioned along segments of the fuselage approximately perpendicular to a longitudinal axis X of the aircraft. The stringers 25 extend over the panels approximately along the longitudinal axis X.
The floor 4 is a primary structure inside the body structure 2 of the fuselage. A floor segment is constructed from an assembly of cross members 41 and rails 42, 43. The cross members 41 are generally straight and horizontal, in an aircraft reference frame, and extend perpendicular to the longitudinal axis X along an axis Y of the aircraft. Their role is to channel the forces related to the load on the floor towards the fuselage body structure. The rails 42, 43 extend along the longitudinal axis X. They are used to fix pieces of furniture, such as seats or monuments.
Among the rails, rails 43, known as false rails as they are not used to fix cabin items on the floor, are installed close to frame 23-cross member 41 connectors to strengthen the structure.
Many connections are required to produce the body structure 2 and the floor 4 assembly.
Thus in order to channel forces (mostly the weight of a load on the floor) to the fuselage body structure, the cross members 41 are fixed to the frames 23. In a specific segment Y of the fuselage, the cross member 41 located in this segment is therefore fixed to the frame 23 located in the same segment Y of the fuselage, firstly to one end and secondly by means of a brace to another point (52) of the frame.
In addition, to stabilize the floor and absorb the energy generated by a deceleration along the longitudinal axis X in the event of a crash, anti-crash rods 6 connect firstly, at a first end 61, a stringer 25 and secondly, at a second end 62 opposite the first end 61, the false rail 43.
These multiple connections between the floor and the fuselage body structure mean that the floor must to be added to the fuselage while the body structure is not yet closed circumferentially.
Effectively, the floor must form a reference surface that is flat and perfectly aligned with the aircraft's XY axes. Given the large dimensions of elements present it should therefore preferably be pre-assembled at the scale of the section and integrated to it as a constructed sub-assembly. Integrating the floor in the aircraft cockpit cross member by cross member would entail complex adjustments and would be economically unfavorable.
In addition, each floor cross member must be matched to a frame in order to be fixed to it. This relative positioning of the pre-assembled floor with respect to the body structure is statically indeterminate and requires a systematic realization of both the floor and the fuselage. The cross members and the frames are structural elements designed to channel significant force flows and therefore both are very rigid. As a result, these elements are not able to deform in order to adapt to any misalignment. It is therefore preferable to assemble the floor with a lower portion of the body structure, known as the lower tub, before said lower section receives an upper portion of the body structure, known as the upper roof section, which closes the body structure.
One known method for assembling a fuselage section comprising a body structure inside of which a floor is attached consists of:                aligning the floor 4 on a special tool,        positioning frame sectors and then panels forming the sides of the floor on said frame sectors,        bonding and fixing the lower tub on the frame sectors, the tub itself being constructed by the assembly of stiffened panels,        bonding the upper portion, known as the roof, on the tub assembled to the floor and thus closing the structure,        installing and assembling the braces.        
According to this method, the fuselage's body structure is constructed from stiffened panels assembled into subsets around the previously produced floor. Each cross member is individually linked at each of its ends to the body structure by an embedding connection. Embedding refers to a complete connection able to transmit all the forces and all the moments in the 3 spatial directions. This embedding connection stabilizes each cross member individually particularly with respect to modes of bending according to X or Z axis moments and with respect to modes of torsion and buckling according to Y axis moments.
The constant improvement in aircraft performance today spurs the increasingly common use of structural elements (panels, frames, stringers) made of composite material for the aircraft's fuselage structure, due to the weight reductions that can be obtained for such structures with these composite materials.
The use of composite materials for the manufacture of fuselage structures allows a one-piece fuselage body structure to be produced, known as a full-barrel composite fuselage. The floor and the body structure can therefore no longer be assembled to form a fuselage section by the method currently utilized and described above.
Moreover, putting a previously assembled floor in place in the circumferentially closed body structure increases the difficulties linked to the manufacturing tolerances and static indeterminacy of the installation, since the body structure thus closed is extremely rigid and misalignments cannot be compensated for by the structure's elastic deformation.
There is therefore a need for a fuselage structure able to allow a previously constructed floor to be assembled economically in a circumferentially closed body structure, and to allow slight misalignments between the floor cross members and frames to be compensated for.
A composite floor piece able to be inserted into a fuselage section forming a circumferentially closed cockpit is known from U.S. Pat. No. 4,479,621. Said floor is constructed from an assembly of one-piece plates comprising longitudinal stiffeners extending parallel to the X axis of the fuselage enclosed between two plates made of a composite material. Such a floor has a smaller width than the interior diameter of the fuselage measured at the tops of the frames and can therefore be introduced into the constructed section, plate by plate. It is then linked structurally to the body structure through articulated rods that allow slight misalignments between the floor's attachment points and the frames' points for spreading forces to be compensated for. This technical solution of the prior state of the art has two drawbacks:                if the floor structure experiences deterioration during the aircraft's operation, repairs are complex because they entail changing the entire plate that has undergone the deterioration and thus extracting it from the fuselage.        the relative movements of the parts at the joints of the rod connections, combined with contact with humidity and corrosive chemicals, particularly in the floor area, result in deterioration problems for said joints through fretting and fretting corrosion.        
The other solutions known from the prior state of the art, for example in German patent application DE 10 2005 045 181 A or in European patent applications EP 1 614 625 A and EP 2 030 891 A also make use of rod connections and therefore suffer the same risk of deterioration of these connections by friction and corrosion or use one-piece floor elements, for example in international application WO 2008 1097711 A, which therefore suffer the disadvantages cited above concerning the possibility of repair.